GENERAL DESCRIPTION
The 1570B Amplifier is a compact, high quality, 165 watt power amplifier designed for public address applications, either
shelf or rack mount. Rack mounting occupies six units (IO1/2") of rack space. Negative feedback is carried around all stages
from a tertiary winding on the heavy duty output transformer, permitting the output to feed an ungrounded load or a load
with one side grounded. This amplifier is designed for stable 70 volt line operation under all output load conditions without
impairment of program quality. Power switch, pilot light and gain control are located on the front of the amplifier and the
input and output terminals, in the form of barrier type terminal blocks, are mounted on the rear. A six-foot three-wire power
cord, terminating in a three-pin cap, is standard equipment with this amplifier.
A special protective device is incorporated in the 1570B in the form of a thermal circuit breaker shunted by a power resistor,
the combination being in series with one side of the power transformer primary. The breaker is fastened to the amplifier
chassis so that heat from the latter is transferred to the breaker thermal element. AC line current through the breaker also
heats the thermal element. Abnormal heating of the chassis and abnormal line current will make the breaker contacts open,
inserting the power resistor in series with the power transformer primary reducing the voltage to it. Program service is thus
maintained at reduced power. The breaker being an automatic reset type, reduction in line current and chassis temperature
will allow it to close and restore full line voltage to the power transformer. Cycling will continue until the abnormal condi-
tion is corrected. Locating the amplifier in an inadequately ventilated, high ambient temperature zone will result in excessive
heating. Overdriving the amplifier due to a mismatch of load to amplifier output will result in abnormal line current.
INSTALLATION
When mounting 1570B Amplifiers in a standard rack or cabinet rack, use a minimum of one rack unit (1 3/4") spacing between
amplifiers and fill space with the 10399 perforated panel. When cabinet racks are used, a blower of approximately 40 cfm
capacity should be installed on top of the cabinet to exhaust hot air from within the cabinet. Fresh air is thus drawn into
the amplifier chassis through the front grill openings.
INPUT CONNECTIONS
Two pairs of input terminals are provided. Terminals 1 and 2, which connect directly to the input potentiometer, are pro-
vided for unbalanced high impedance sources or for bridging unbalanced low impedance lines having a signal voltage
of 1 volt or more. Terminals 3 and 4 connect to a standard octal socket which permits low impedance input from balanced or
unbalanced lines of 150 or 600 ohms with the accessory 15095 (plug-in) Line Transformer. The socket is connected for 600
ohm operation as shipped. The 150 ohm input is obtained by strapping the socket terminals as shown on the schematic.
Both inputs may be used simultaneously provided the input to 1 and 2 is built out with a 100,000 ohm resistor in series with
the high side of the line to prevent excessive loading or shorting of the low impedance input.
OUTPUT CONNECTIONS
Output taps for nominal loads of 8, 16, and 32 ohms (70 volt line) are provided. Connections and strapping are shown
below.
Speaker Matching:
Use the output tap which most nearly equals the total speaker impedance. If the load impedance
falls between two output tap values, use the lower tap.
70 Volt Line:
The constant voltage distribution system (70 volt line) permits connection of a large number of speakers,
each to operate at the power level desired, without regard for the impedance involved. In this system, each speaker is
equipped with a transformer which has a number of taps rated in terms of power and the tap is selected which gives the
desired speaker power. The total power required for the speakers should be equal to or less than the amplifier power rating.
Protection of Horn Loaded Drivers:
In industrial paging systems, stadium, arena or other voice reinforcing systems
which require excellent intelligibility over high noise levels, diaphragm type driver units coupled to horns are used. When
these are used without loudspeaker dividing networks (used in two-way speaker systems), the low frequency energy applied
to the driver voice coils must be limited. This protection is provided in the 1570B Amplifier by means of an R-C high pass
filter in V-1 grid circuit (see schematic). As shipped, capacitors C9 and C10 are strapped out. By cutting one or both of
these straps attenuation is introduced as shown in the table, depending on the impedance of the source.
CPS
Source Impedance
Cut Strap
250
500
1000
2000
100,000 Ohm
B
-4.0 db
-1.5 db
-.5 db
-.2 d b
A
- 3 . 0
-1.0
-.3
-.2
A and B
-7.0
-2.5
-.8
-.3
Low
B
-5.2
- 2 . 0
-.7
-.2
(Approximately
A
-8.5
- 4 . 0
-1.5
-.5
correct to
15,000 ohms)
A and B
-12.5
-6.2
-2.5
-.7
Summary of Contents for 1570B
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